Is Climate Change Causing A Rise In Mosquito And Tick-Borne Diseases?

Life in a single-family home in suburban America, one with a quiet and spacious backyard, surrounded by natural habitats, lush green vegetation, where beautiful birds, squirrels and other small mammals come and go, is the American dream. Now, however, this once-cherished dream is being threatened by invading tick and mosquito species that are carrying emerging pathogens.

Public health officials use emerging vector-borne diseases (VBDs) in this context to refer to diseases or pathogenic agents transmitted by mosquitoes and ticks that have recently entered human populations for the first time. In some cases these diseases have historically been present in humans but have increased in frequency, geographic range or both.

This finding is particularly important, because climate change scenarios predict ENSO-like events to become more frequent. This is also exemplified by the recent global spread of Zika virus, where extreme climatic conditions during one month in several parts of South America in 2015 were followed by the spread of Zika virus to that location the next month.

Climate change and disease vectors

Climate change can shape the rates of emerging disease by various processes including a direct effect on mosquito and tick vectors, and indirect effects on human vulnerability to emerging VBDs. For example, warmer temperatures lead to longer breeding seasons and greater hatch rates, especially for mosquito populations. This will cause these vectors to seek more territory, which is also more readily available from warmer temperatures.

The environments characteristic of most suburban residential properties are enticing habitat for these vectors, which might explain why more ticks and human infections with emerging VBDs are increasinglyreportedfrom residential areas.

Indirect effects of warmer temperatures include weakening human immune systems after droughts or flooding, and disruptions to the health care system following disasters such as hurricanes and floods. Many scientists consider this indirect mechanism, where exceptional climatic conditions affect people’s behavior and increase exposure to vectors, to explain much of how climate change affects the spread of emerging VBDs.

For scientists to say definitively that climate change is directly boosting the population of disease-carrying mosquitoes and ticks, they would first need to demonstrate that the changes in distribution of these disease vectors were not due to other factors.

Why should we care about the association between climate change and the spread of emerging VBDs? For one, common bugs can transmit multiple pathogens. For example, the mosquito Aedes aegypti alone can spread Zika virus, dengue virus, chikungunya virus and yellow fever virus, while the tick Ixodes scapularis alone can transmit the causative agents of Lyme disease, anaplasmosis and babesiosis, among others.

Secondly, many of these diseases lead to increased health care costs, disability, and even death for affected individuals.

So far the current evidence suggests that while climate change is an important factor in the geographic and seasonal variation in mosquito- and tick-borne diseases, other factors including land-use patterns, socioeconomic and cultural factors, pest control, access to health care and human response to disease risk also play a role.

More rigorous long-term studies are needed to demonstrate unequivocally the impact of climate change on emerging VBDs of public health importance. If climate change increases the transmission of these diseases, we need to take all necessary steps to understand how this occurs with a view to preventing it. Otherwise, the American dream of home ownership in the suburbs is threatened, and climate change may soon be added to the long list of injustices and challenges that have undermined this American dream.